Nature has endowed human beings with two ears so that our hearing is binaural. Early musical reproductions were monaural, which was analogous to going through life with one ear covered. Thus, sensations of spaciousness (width and depth) and imaging (location) were lost. These sensations are perceived by the brain as a result of subtle differences in sound amplitude and phase reaching our ears. Such sensations help us orient ourselves by sound even without the aid of our eyes and gives sound a natural character even when reproduced artificially.
To explain further how binaural hearing gives sound a "natural character", consider a large concert hall with a full orchestra on a stage at one end thereof, and a listener seated facing the orchestra towards the other end thereof. As the instruments on the listener's left side produce sound waves, these sound waves reach the listener's left ear slightly sooner in time than they reach the listeners right ear. This slight difference in time (which may be on the order of only a millisecond), between the sound as heard with the left ear and the same sound as heard by the right ear, enables the listener to discern even without looking (i.e., even if blindfolded), that the sound is coming from the left side of the stage. Thus, if the sound wave is heard by one ear slightly before the same sound wave is heard by the other ear, this difference in time (commonly referred to as a "phase change"), helps the listener identify the "location" or "image" of the sound source. Similarly, if the sound wave is heard by both ears at the same time, then the listener can ascertain that the source of the sound is more or less at the center of the stage. Likewise, if a different second sound wave (e.g., from a different source) is heard by both ears at approximately the same time but slightly delayed from an earlier first sound wave, then the listener knows that the source of the second sound wave is positioned behind the source of the first sound wave on the stage. If a corresponding phase change accompanies the reception of these first and second signals, the listener is further able to determine whether these first and second sources are positioned to the left or right on the stage. Hence, when all of the sound waves emanating from all of the orchestral instruments placed all around the stage are combined, the listener knows (even without the aid of sight) that the orchestra has "width" and "depth" (spaciousness) associated therewith; and is further able to "locate" or "image" where selected instruments (sources of sound) are placed on the stage, especially if the listener can readily "pick out" or discern the sound of one instrument from the others.
In contrast, if the source of the sound waves is emanating from a single point source, such as a speaker of a monaural sound system, the sound heard, regardless of the fidelity or signal integrity thereof, is not natural because it has no discernable depth or width associated therewith. Further, imaging is not possible, because the sound waves all emanate from the same location. While stereo and other multi-channel systems (the "stereo effect") improve the quality of the sound a great deal, the sound is still not truly natural in the sense that the imagining and depth/width that are achieved are highly dependent upon the placement of the speakers and the position of the listener relative thereto.
Prior art systems which simulate or synthesize stereo from a monaural audio source have generally been realized by employing two channel reproductions with the second channel delayed from the first. In addition, positive feedback was added which causes an effect similar to reverberation and thus enhances the results. However, most of these devices have failed to achieve any commercial success because a substantial portion of the frequency response has been lost in the attempt to synthesize the stereo effect. This occurs partially because of the very fact that the signals are substantially out of phase. A similar effect can be heard by connecting out of phase speakers to a stereo source. A substantial loss of spaciousness and imaging as well as fidelity can be perceived.
Another prior art device attempts to simulate stereo by splitting frequencies or shunting audio signals in and out of phase, mostly in a lead and lag situation. One such device is disclosed in U.S. Pat. No. 3,670,106 of Orban. This device takes an input source and splits it into five bands, then changes the phases of each band according to frequency to produce a stereo effect. However, by splitting frequencies a certain amount of music information is lost where these frequencies cross over because of an out-of-phase condition. Further, when summing amps are used to put the signal together again, there will be another loss in separation.
Another device, disclosed in U.S. Pat. No. 3,156,769 of Markowitz, uses incandescent lights which drive a photoresistor to shunt the input signals to a power amplifier. In this circuit, the incandescent bulbs or lights are somewhat slow in reacting and are only good for a certain period of time. As the light gets increasing carbon buildup on the glass envelope, its intensity will change, resulting in a change in impedance out of the photoresistor for shunting ability. Thus, while this circuit will in effect simulate two separate channels, there is a great loss of quality (i.e., clarity and fidelity) of the sound reproduction because of the difference in phase as well as the loss of some frequencies.